Thermal overload relays have been known heretofore. For example, P. C. Fryer U.S. Pat. No. 4,096,465, dated June 20, 1978, shows a thermal overload relay intended to be connected in a three-phase power supply to a three-phase motor which includes means responsive to the currents in the individual phases and a contact-breaking switch arranged to be actuated by the current responsive means in the event of the current on all three phases exceeding a threshold and also in the event of a differential, above a second threshold, and with the magnitude of the second, or differential threshold increasing with the mean value of the currents of all phases.
While prior thermal overload relays such as that represented by the Fryer patent have been useful for their intended purposes, this invention relates to improvements thereover.